CONCEPTS IN BIOLOGY

PART V. THE ORIGIN AND CLASSIFICATION OF LIFE

23. The Animal Kingdom

23.11. Arthropods

The arthropods are the most successful group of animals on Earth. The earliest arthropods were marine, and today arthropods are found as plankton, nekton, and benthic animals. However, the most successful group of animals is the terrestrial insects. Over three-fourths of the species of animals are arthropods, and over three-fourths of the arthropods are insects.

Arthropods have an external skeleton composed of chitin. Their bodies are segmented, but the segmentation is highly modified, compared with that of the annelids. There is generally a head region, and the segments that follow may be modified into regions as well. Many of the segments have paired appendages. Both the body and the appendages are jointed.

Because they have an exoskeleton, in order to grow they must shed their skeleton and manufacture a new, larger one at intervals. They have well-developed nervous, muscular, respiratory, circulatory, and reproductive systems (figure 23.23).

FIGURE 23.23. Arthropod Structure—Grasshopper

(a) Arthropods have an external skeleton and segments that are modified into special body regions. They have jointed appendages. (b) They have complex organ systems.

There are many types of arthropods: crustaceans, millipedes, centipedes, arachnids, and insects (figure 23.24). Nearly all crustaceans are aquatic (both marine and freshwater) but range in size from large crabs and lobsters to tiny, planktonic organisms and sessile barnacles. Crustaceans are typically omnivores, which feed on a variety of living and dead materials.

FIGURE 23.24. Arthropod Diversity

Arthropods are the most successful group of organisms on Earth.

Millipedes and centipedes have long bodies with many legs, and all the posterior segments are similar to one another. The earliest fossils of terrestrial animals are of this type.

The arachnids are the scorpions, spiders, mites, and ticks. They are very successful as land animals. Mites are extremely common as soil organisms. Spiders are carnivores, and most ticks are blood feeders.

Insects are terrestrial animals with a body divided into three regions: head, thorax, and abdomen. They have three pairs of legs, and most species have wings.